As technology to protect the privacy of a person captured as the subject in an image, image processing is known in which a mosaic is applied to the face region of the person in an image, or the eye region of the person is masked with a rectangular shape, so that the person is rendered unidentifiable. However, images that have had such image processing performed on them are images that can be clearly determined at a glance to have been doctored, and they create an un-natural impression for a viewer of these images.
In order to resolve this issue there has been a proposal for a technology in which a mask image is synthesized at a synthesis target region such as a face region in an image by employing gradient data of a mask image to solve a Poisson equation, with the edge of the synthesis target region (the synthesis boundary with the mask image) employed as a boundary condition. In such technology, an initial value image is applied to the synthesis target region, and then iterative computation is performed until differences of each the pixels from the initial value image converge to values that satisfy the gradient data of the mask image and the boundary condition of the edge, thereby generating a synthesized result image that corresponds to a result of synthesizing the mask image at the synthesis target region.
Note that when the synthesis target region is the face region, an image representing the face of another person or an image representing an average face of plural persons may, for example, be employed as the mask image. Moreover, an image in which the whole area is whited out, or an image in which the whole area is blacked out may, for example, be employed as the initial value image.